1. Field of the Invention
This invention relates to a method of projecting a line image of uniform brightness and more particularly to a method of projecting a laser beam line image having uniform brightness over the entire length thereof.
2. Description of the Prior Art
As one method for inspecting the surface eveness or the uniformity of thickness of web material, there has been known a method in which a line image is projected on the surface of the web material being transported as between two rolls. The line image is projected onto the surface in the direction normal to the direction of transport of the web material. The light reflected by the surface carries various information about the condition of the surface. For example, when the light is projected on a screen, the projected image of the light will be a straight line if the surface of the web material is completely flat across its width, while if the surface is not flat the projected image will be a curved line. Further, if there are scratches on some area of the surface, the brightness of the corresponding area of the projected image will be lowered. The projected image of the reflected light may be photographically recorded on a photosensitive film strip which is transported synchronously with the web material.
There has been known a method of projecting the line image utilizing a laser beam. In the conventional method, a laser beam emitted from a laser is first collimated by a collimator lens and then the collimated laser beam is diverged by a cylindrical lens to project onto a surface to be inspected.
The conventional method is disadvantageous in that the brightness of the projected line image is not uniform over the entire length thereof, since the laser beam emitted from the laser has a Gaussian distribution of intensity about its optical axis, and accordingly the intensity of the diverged beam is highest at its optical axis and is gradually lower toward its end portions. Therefore, the brightness of the projected line image is inherently highest at its center and lowest at its ends.